Transmission diversity with two cross-polarised antennas arrays

ABSTRACT

A method for selecting a diversity mode to be applied by a transmitter having two cross-polarized antenna arrays, each representing a diversity branch, comprising the steps of: providing a plurality of diversity mode performance chart look-up tables, mapping a respective individual diversity mode to a respective pair of time correlation value and space correlation value for the two cross-polarized antenna array beams, wherein a respective individual diversity mode is presented by a mapping area, wherein the plurality of performance chart look-up tables is parameterized by an indication of a ratio of received powers from the diversity branches, first selecting one of the performance chart look-up tables dependent on determined ratio of received powers from separate beams, and second selecting one of the individual diversity modes according to the mapping to the determined actual time relation and space correlation values from the first selected performance chart look-up table.

FIELD OF THE INVENTION

The present invention relates to a method for selecting a diversity modeto be applied by a transmitter having two cross-polarized antennaarrays.

BACKGROUND OF THE INVENTION

In detail, the present invention relates to the field of transmitantenna diversity. A rather comprehensive introduction to the technicalfield of diversity is for example given by Juha Korhonen in“Introduction to 3G mobile communications”, chapter 3.4., page 86 to 94,Artech House mobile communications series, 2001. More specifically, theconcern of this invention resides in a mode selection procedure by whicha suitable transmit diversity mode for each link in the cell defined bythe coverage area of the subject transmitter can be selected.

It is to be noted that while reference is made to an antenna array, anarray comprising a single antenna only may still be regarded as anantenna array. Likewise, an antenna and/or antenna element of the arrayproduces a beam of electromagnetic radiation in operation, i.e. whenbeing driven, and also terms “antenna” and “beam” are usedinterchangeable, as the antenna configuration and driving will determinethe produced beam. It is to be noted further that when consideringdiversity, a diversity branch can also be represented by the beamproduced by the corresponding (diversity) antenna array.

To recapitulate, there are different transmit diversity modes in FDDWCDMA (FDD=Frequency Division Duplex, WCDMA=Wideband Code DivisionalMultiple Access) dedicated downlink channels:

-   1) an open-loop diversity mode using space-time codes such as for    example the concept known as STTD (Space-Time Transmit Diversity),    and-   2) a closed-loop diversity mode, which can be classified into    different closed-loop diversity classes:-   a) a first class, subsequently referred to as class 1, in which the    receiver (e.g. a user equipment UE) returns information to the    diversity transmitter (e.g. a Node_B) concerning the relative phase    of the received diversity transmission signals; an example for such    a class 1 closed-Loop diversity mode is known as closed-loop loop    mode 1; and-   b) a second class, subsequently referred to as class 2, in which the    receiver returns information to the diversity transmitter concerning    the relative phase and the ratio of received powers of the received    diversity transmission signals; an example for such a class 2    closed-loop diversity mode is known as closed-loop mode 2 (as for    example described in the above cited book by Juha Korhonen or as    described by 3GPP TS 25.214: “Physical Layer Procedures (FDD)”).

It is to be noted that closed-loop diversity is only applicable to adownlink channel if there is an associated uplink channel which isrequired to return the feedback information. An example for such achannel combination is represented by the DPCH (Dedicated PhysicalCHannel)/DPCCH (Dedicated Physical Control CHannel) channels.

First, the present transmit diversity methods in WCDMA are recalled. Thepresent (standardized) Transmit Diversity Methods in FDD WCDMA dedicateddownlink channels are, as mentioned herein before, for example

-   STTD (Space-Time Transmit Diversity) as an open-loop solution    utilizing a simple 2×2 space-time code,-   closed-loop mode 1 (CL1) as an example for class 1 closed-loop    diversity according to which the relative phase between transmitted    signals is adjusted based on the feedback from the receiver (user    equipment), and-   in closed-loop mode 2 (CL2) as an example for class 2 closed-loop    diversity according to which both, relative phase and power between    transmitted signals, are adjusted based on the feedback from the    mobile.

The Node_B as the transmitter (corresponding in its functionality to abase station BS known from GSM system) can select the mode to be usedfor each link separately or it can use the same mode for all links inthe cell. In this invention, however, the former case is studied, sincethe latter solution is not recommended because it does not utilize theavailable capacity potential.

Generally, the mode selection problem resides in finding measures and amethod by which a best suitable mode can be selected for each link. A“best suitable mode” can be determined as the mode which reveals thebest performance (e.g. lowest bit error rate, lowest S/N (signal tonoise) ratio, or the like).

The mode selection problem is linked to the employed antenna solution atthe transmitter. The present transmit diversity modes are designed byassuming that the average received powers at the receiver side (UE) fromseparate transmitter (Node_B) antennas are the same. Thus, if averagepower of signals received at the UE and originating from an antenna Ant1is denoted by P1 and average power of signals received at the UH andoriginating from antenna Ant2 is denoted by P2, so that the ratiothereof, P1/P2=1. When this assumption holds, the performance of theclosed-loop schemes depends in a certain manner on the feedback delayand on the spatial correlation between transmit antennas. If the ratioP1/P2 is not equal to unity, then the sensitivity of closed-loop modesto the feedback delay and spatial correlation between transmit antennaswill be different than in case P1/P2=1. This can be illustrated by asimple example. If transmitter selection combining is used, then thesignal is transmitted trough the antenna which provides better channel.Feedback information from a mobile directs the transmit antennaselection. If P1/P2=1 in the receiver, then both received channels arein good state with equal probability and system performance depends onthe feedback delay and transmit antenna correlation. If feedback delayis large when compared to channel coherence time, then systemperformance is corrupted. Similarly, if transmit antennas correlateheavily, then both antennas are in good and bad states simultaneouslyand the lack of diversity corrupts the system performance. However, ifP1/P2 is all the time very high—or very low—then the same antennaprovides better channel almost all the time and feedback delay orantenna correlation are not corrupting the system performance much. Theeffect of P1/P2 is not the same to all transmit diversity modes andtherefore it should be taken into account when transmit diversity modeis selected.

In FIG. 1 there is a sketch concerning to the selection problem, whenaverage received powers from separate BS antennas in MS are equal: areasA, B and C (also referred to as Mapping areas) consist of thosespatio-temporal correlation value pairs (spatial correlation SC,temporal correlation TC), for which open-loop mode (e.g. STTD), class 1closed-loop mode (e.g. CL1) and class 2 closed-loop mode (e.g. CL2),respectively, provide the best performance. Apparently, the open-loopmode (STTD) is the best choice in most cases while the operating area ofclass 2 closed-mode is marginal: class 2 is suitable on ly when timecorrelation (TC) is very high. class 1 works well when spatialcorrelation (SC) is relatively high. It is to be noted that the FIG. 1is only a sketch.

The assumption that average received powers at the receiver would beequal is well posed, if

-   (a) Transmit antennas are co-polarized-   (b) Transmit antenna separation is not very large (i.e. antennas,    are in the same site, and not, for example, in separate buildings).

The latter restriction (b) is not a problem in practice since thediversity antennas are usually in the same mast. However, assumption (a)has some drawbacks.

The well known fact is that two low correlated signals can be obtainedusing cross-polarized antennas (and/or antenna arrays). This antennasolution is more compact and cheaper than a pair of spatially separatedco-polarized antennas/antenna arrays. Moreover, beside these knownadvantages, the so-called polarization mismatch can be avoided by usingcross-polarized antenna arrays antennas. This is explained in thefollowing.

Polarization Mismatch Problem:

In a conventional system, a single vertically polarized antenna is usedat the transmitter side. This arrangement is feasible if receiverantennas are all vertically polarized. However, this will not be thecase in practice. It may even happen that receiver antenna polarizationis flat, a horizontally oriented ellipse, or the like. Then hugepolarization mismatch losses can be faced.

Also, different physical environments preserve the transmittedpolarization in a different way and thus, the danger of polarizationmismatch depends on the environment; in open areas the probability of aserious mismatch is expected to be high.

The known solution to this problem is the use of cross-polarizedpolarized antenna arrays and/or antennas. In GSM related solutions, thedownlink (DL) signal is transmitted from cross-polarized antennabranches with equal power (each individual diversity antenna correspondsto a transmitting branch). The relative phase between the signalstransmitted from each branch is randomly rotated in order to avoid thesituation where signals would erase each other for a long time (this mayhappen if antenna brancnes have strong correlation, for example becauseof a line-of-sight, (LOS) situation). This solution prevents the totalmismatch between the transmitter and receiver polarizations.

The same method as used in GSM is, however, not straightforward to beused in WCDMA, since channel estimation at the receiver UE is based oncommon p-lot channel (CPICH). However, if downlink transmit diversity isused, then different CPICHs are transmitted from separate antennabranches and cross-polarized antennas can be employed. Stated in otherwords, via each antenna of the transmit diversity antennas a respectiveCPICH is transmitted.

The present WCDMA standards specify in a detailed manner the allowedtransmit diversity modes and hence, if cross-polarized polarizedantennas are used in connection with transmit diversity, then thespecial properties of this antenna solution must be taken into accountin the limits given by standards.

Summarizing, it has to be noted that

-   (a) if co-polarized, spatially separated antennas are used in    connection with WCDMA transmit diversity modes, then mode selection    can be based on a fixed performance chart such as the one proposed    in FIG. 1;-   (b) based on compact structure, costs and robustness against    polarization mismatch, the use of cross-polarized diversity antennas    is a very attractive solution. However, when used, this renders the    usage of a fixed performance chart such as the one proposed in FIG.    1 not feasible, as will subsequently be explained.    Special Characteristics of Cross-Polarized Antenna Arrays:

Currently, the present transmit diversity modes are studied based on theassumption that the average received powers at the receiver originatingfrom separate transmitter antennas are equal (P1/P2=1). This is,however, not necessarily true for cross-polarized antenna arrays if theXPR (cross-polarization ratio) in the channel is high. This is the caseespecially in rural environments where orthogonal polarization branchesare not mixed well and it has been claimed that XPR is relatively higheven in urban outdoor environments (See for example Shapira J. andMiller S. in “A novel polarization smart antenna”VTC, May 2001, or in“Transmission Considerations for polarization-smart antennas”VTC, May201. See also “Method and System For Improving Communication” by ShapiraJ. and Miller S., International Patent Application WO 98/39856).

Thus, although there might be low correlation between BS antennabranches, the average received power at the receiver fromcross-polarized BS antenna branches can be different (P1≠P2); thisrenders the usage of a fixed performance chart such as the one proposedin FIG. 1 not feasible, or at least its usage will result in anon-optimum diversity mode selection.

SUMMARY OF THE INVENTION

Consequently, it is an object of the present invention to provide animproved method for selecting a diversity mode to be applied by atransmitter having two cross-polarized antenna arrays, each representinga diversity branch, for transmission diversity, which is free from theabove mentioned drawback.

According to the present invention, the above object is for exampleachieved by a method for selecting a diversity mode to be applied by atransmitter having two cross-polarized antenna arrays, each representinga diversity branch, for transmission diversity, the method comprisingthe steps of: providing a plurality of diversity mode performance chartlook-up tables, each performance chart look-up table mapping arespective individual diversity mode out of a plurality of individualdiversity modes to a respective pair of time correlation value and spacecorrelation value for said two cross-polarized antenna array beams,wherein a respective individual diversity mode is presented by a mappingarea, wherein the plurality of performance chart look-up tables isparameterized by an indication of a ratio of received powers from saiddiversity branches, and the mapping is different for differentperformance charts, first determining the ratio of received powers fromsaid diversity branches, second determining the actual rime correlationand space correlation for said pair of two cross-polarized antennaarrays, first selecting one of said performance chart look-up tablesdependent on determined ratio of received powers from separate beams,and second selecting one of said individual diversity modes according tothe mapping to the determined actual time correlation and spacecorrelation values from said first selected performance chart look-uptable.

According to favorable further developments

-   -   said selected diversity mode is selected for each individual        link established by the transmitter;    -   said mapping of diversity modes differs for different        performance chart look-up tables dependent on the determined        ratio of received powers from said diversity branches;    -   said diversity modes are classified as open-loop diversity modes        and closed-loop diversity modes, and said determined ratio of        received powers is applied as a further control parameter for        controlling said closed-loop diversity modes when activated upon        selection;    -   said diversity modes are classified as open-loop diversity modes        and closed-loop diversity modes, and a mapping area of at least        one closed-loop diversity mode increases dependent on the        indication of a ratio of received powers from said diversity        branches;    -   said first determining and said second determining are performed        at said transmitter;    -   providing said performance chart look-up tables is effected        beforehand based on simulation results and/or measurement        cycles; and    -   both arrays consists of one antenna element and antenna        calibration in the transmitter is performed by using both the        feedback from said receiver according to one of the closed-loop        modes, and the received signals from cross-polarized antenna        arrays in the transmitter.

By virtue of the present invention, basically the following advantagescan be achieved

-   -   The present invention provides a new solution for diversity mode        selection to be used in two-antenna array transmit diversity        systems, in particular those using cross polarized antenna        arrays.    -   The present invention provides means to combine polarization        matching and present WCDMA transmit diversity modes.    -   The mode selection procedure depends on the difference between        average received powers, so that the appropriate look-up table        may be selected resulting in improved diversity performance,    -   The present modes (in limits given by standards) can be enhanced        as they take into account the difference between average        received powers.

Thus, this invention improves the present solutions in two ways in thatfirstly, it provides means to combine polarization matching with presenttransmit diversity modes, and secondly, it provides a new transmitdiversity selection procedure that is based on BS measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail with reference to the accompanying drawings, in which

FIG. 1 shows a diversity mode performance chart look-up table mapping arespective individual diversity mode out of a plurality of individualdiversity modes to a respective pair of time correlation value and spacecorrelation value, under the assumption that the ratio of receivedpowers equals 1;

FIG. 2 shows a diversity mode performance chart look-up table mapping arespective individual diversity mode out of a plurality of individualdiversity modes to a respective pair of time correlation value and spacecorrelation value, under the condition that the ratio of received powersis unequal to 1, and rather is in the range of for example 3 to 6 dB;

FIG. 3 shows a diversity mode performance chart look-up table mapping arespective individual diversity mode out of a plurality of individualdiversity modes to a respective pair of time correlation value and spacecorrelation value, under the condition that the ratio of received powersis unequal to 1, and rather is in the range of for example greater than6 dB;

FIG. 4 is a simplified block diagram of a transmitter with the presentinvention being implemented; and

FIG. 5 is a flowchart illustrating the method according to the presentintention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, the proposed mode selection procedure according to thepresent invention is outlined. It is to be noted that in the be lowprocedure all steps are performed in the transmitter (e.g. BS). Theprocedure as such basically comprises the following steps when selectinga diversity mode A, B, C to be applied by a transmitter having twocross-polarized antenna arrays Ant1, Ant 2, each representing adiversity branch, for transmission diversity.

Firstly, a plurality of diversity mode performance chart look-up tablesLUT, LUT1, LUT2, LUT3 are provided, step S10. Each performance chartlook-up table maps a respective individual diversity mode A, B, C out ofa plurality of individual diversity modes to a respective pair of timecorrelation value TC and space correlation value SS for said twocross-polarized antennas, and a respective individual diversity mode ispresented by a mapping, area (in the chart). The plurality ofperformance chart look-up tables is parameterized by an indication of aratio P1/P2 of received powers (at the receiver side) from saiddiversity branches, and the mapping is different for differentperformance charts. The look-up tables have been formed beforehand, i.e.before the actual selection process starts.

Subsequently, there is a first determining step, S11, for determiningthe ratio of received powers from said diversity branches. Thisdetermination can be effected by computing the ratio between averagereceived powers from separate antenna branches. Namely, the receiver UEtransmits in uplink direction UL feedback information FBI indicating therelative phase of the signals transmitted/received received from theantenna branches, and from this information, the transmitter can in turndeduce the relative strength, i.e. received power. The average receivedpower is obtained by averaging the information on the relativestrength/power received from a plurality of receivers (mobile stationand/or user equipment), as each FBI concerns a certain receiver only.

Next, in a second determining step S12, the actual time correlation TCand space correlation SC for said pair of cross-polarized antennas isperformed. This can be accomplished by computing the channel timecorrelation employing the data from both receiver antenna branches byusing any known method, and likewise, also by computing the spatialcorrelation between antennas by using received data and any knownmethod. The computation of the time correlation of signals can be done,for example, in the transmitter based on the equation

${{TC}(d)} = \frac{E\left\{ {{x(t)}^{*}{x\left( {t - d} \right)}} \right\}}{\sqrt{E\left\{ {{x(t)}}^{2} \right\} E\left\{ {{x\left( {t - d} \right)}}^{2} \right\}}}$where x(t) is the received signal in the transmitter at time instant t,x(t−d) is the received signal in the transmitter at time instant t−d andE {·} is the expectation. Expectations can be estimated using IIR or FIRfilters. The symbol “*” denotes the conjugate complex value. Thecorrelation between antenna branches can be done based on the equation

${SC} = \frac{E\left\{ {x\; 1(t)^{*}x\; 2(t)} \right\}}{\sqrt{E\left\{ {{x\; 1(t)}}^{2} \right\} E\left\{ {{x\; 2(t)}}^{2} \right\}}}$where x1(t) is the received signal in the transmitter antenna 1 at timeinstant t, x2(t) is the received signal in the transmitter antenna 2 attime instant t and E{·} is the expectation. Again, suitable IIR or FIRfilters can be used in order to estimate the expectations, and thesymbol “*” denotes the conjugate complex value.

Based on those spatio-temporal correlation values and power ratiodetermined as described above, a decision can then be taken on the mostsuitable diversity mode to be used and/or activated for diversitytransmission, the decision using the look-up table LUT consisting ofplural individual look-up tables LUT1, LUT2, LUT3 that have been formedbeforehand.

Namely, there is performed a first selecting step S13 of one of saidperformance chart look-up tables dependent on determined ratio P1/P2 ofreceived powers, i.e. for example LUT2 is selected because the ratioP1/P2 is in the range of 3 . . . 6 dB for which LUT2 is applicable, andthereafter a second selecting step S14 of one of said individualdiversity modes A, B, C according to the mapping to the determinedactual time correlation TC and space correlation SC values from saidfirst selected performance chart look-up table is performed. Stated inother words, considering that LUT2 resembles the performance chart shownin FIG. 2, assuming that (TC;SC) has a value of about (0,9;0,9), forexample, there the value pair lies within the mapping area denoted byarrow C and the closed-loop diversity mode of class 2 would be selected.

Thus, the principle of mode selection has become clear from the aboveexplanations of the provisioning, determination and selection steps.Known algorithms can be used in the determination steps while theprovisioned look-up table can be established based on simulations and/orthe transmitter tests.

FIG. 5 represents the above explained method steps as a correspondingflowchart diagram.

Accordingly, the mode selection procedure according to the presentinvention can be based, as derivable from FIGS. 1, 2 and 3, on threemeasures and/or parameters that can be used in mode selection, namely:time-correlation correlation TC between signals from separate antennas,spatial correlation SC between separate antennas and the ratio betweenaverage received signal powers P1/P2. There are, of course othermeasures too, but the inventors emphasize the given measures since

-   they are essential from a performance point of view,-   they can be computed in BS and-   most (or even all) other important measures (MS velocity etc.) can    be derived from the given measurement set.

According to a modification, the closed-loop diversity mode of class 2can be enhanced. Namely, as mentioned before, said diversity modes axeclassified as open-loop diversity modes A, and closed-loop diversitymodes B, C, and said determined ratio of received powers P1/P2,according to the modification, is applied as a further control parameterfor controlling said closed-loop diversity modes when activated uponselection.

This will subsequently be explained in greater detail. In the presentclass 2 closed-loop modes, the receiver (MS or UE) estimates thechannels from two different CPICHs and selects the best transmit weightcombination from a standardized quantization set. Then receivertransmits the feedback information (4 bits) to the BS via a controlchannel (e.g. DPCCH). In the transmitter such as the BS or Node_B, thesuitable weights are selected and/or generated based on the feedback.Finally, the receiver MS may use a certain verification algorithm bywhich it confirms that the transmitter BS has used correct weights (seefor example R1-00-1087, verification algorithm for closed-loop transmitdiversity mode 2, TSG ELAN WG1, August, 2000). If there are feedback biterrors, then weights used by the transmitter BS are not those that wereasked by the receiver MS, but verification algorithm will find this outand correspondingly change its weights (in channel estimation based onthe common pilots, i.e. CPICH's).

If, however, cross-polarized antenna arrays (and/or antennas) are used,then the following enhancement can be used.

-   1. The receiver estimates the channel, forms the feedback and    transmits it to the transmitter as in the present closed-loop mode    of class 2.-   2. The Transmitter uses the standardized quantization in weight    selection. However, it does not use the receiver feedback directly,    but instead, it employs both the feedback received from the receiver    as well as uplink measurements in connection with weight selection.-   3. MS verifies the weights that are used by ES.-   4. BS verifies that employed weights are suitable.

Various possibilities in regard of detailed algorithm proposals for suchan enhanced control can be presented, which may vary dependent onmeasurement campaign results conducted for specific environments andtransmitters. However, basically it should be noted that the receivedpower and/or average received power is to be used as are additionalcontrol parameter for weight selection in diversity transmission.

According to a basic implementation of the enhanced modification, thetransmitter BS/Node_B computes the average received powers P1 and P2 ofsome certain link from separate antenna branches and uses the ratioP1/P2 in weight selection. The weight computation can also be based onthe relative phase information received via the FBI bits from a certainreceiver. The received FBI information concerning to a certain radiolink can be either used directly or it can be filtered before it isused.

According to an alternative implementation of this aspect, in manyenvironments the amplitude (and in some cases even phase) weightselection can be based on uplink measurements when cross-polarizedantennas are used. For example, received signal powers from separateantennas can be filtered as follows,P1(t)=aP10+(1−a)P1(t−1)P2(t)=aP20+(1−a)P2(t−1)where P1(t) is the filtered power from antenna 1 at time t, P10 is theinstant power from antenna 1, P1(t−1) is the filtered power from antenna1 at time t−1, P2(t) is the filtered power from antenna 2 at time t, P20is the instant power from antenna 2, P2(t−1) is the filtered power fromantenna 2 at time t−1 and a is a filtering parameter. When the channelpreserves the transmitted polarization, it can be seen from uplinkmeasurements when observing the ratio p1/p2: If the ratio p1/p2 is notequal to unity, then channel preserves the transmitted polarization atleast partly and it can be utilized when downlink transmit weights areselected. In some cases the channel preserves also the relative phasebetween transmitted (and received) signals.

The transmitter verification can be based on the averaged FPC (FastPower Control) bits that are received from the receiver. Thus, theproposed enhanced class 2 closed-loop diversity mode does not need anystandard changes. The performance can be further improved by extendingthe definition of the field of applicability of the class 2 closed-loopdiversity mode in the standard. Namely, the standard commands to send80% of the power from the better channel and 20% of the power from theother one. However, if the power imbalance is large, it is better totransmit from one antenna only. Stated in other words, depending on athreshold of power imbalance, diversity is then switched off. Optionaltransmit weight verification in the receiver side should still be viableif the changes in power allocation based on uplink measurements aresmooth enough.

Next the benefit obtained by the proposed enhancement is illustrated.Assume that cross-polarized antenna arrays are used and ratio p1/p2 ofreceived powers from separate polarization branches is relatively large(currently, there are no simulation results yet available and thereforeFIG. 2 is based on theoretical considerations). It is assumed that here‘relatively large’ means 3–6 dB's). Then the performance chart has theform given in FIG. 2 (now C refer to enhanced class 2). It is seen thatthe operating area of enhanced class 2 has been increased. This ismainly because enhanced class 2 begins to work better when ratio p1/p2increases. Moreover, there are not as many wrong decisions correspondingto amplitude weights when enhanced class 2 is used. This is especiallytrue when spatial correlation is high. In FIG. 3 there is illustratedthe performance chart when the ratio of received powers from separatepolarization branches is large (>6 dB's). It is seen that enhanced class2 begins to dominate when power ratio increases. In fact, when ratiop1/p2 is (very) large (6 dB corresponds to a ratio of P1/P2=4/1), thenenhanced class 2 may be the most feasible mode for all spatio-temporaltemporal correlation values. This is, however, an extreme case.

When enhanced class 2 is used, transmit weight selection may be based onboth receiver feedback and uplink measurements. This means that antennasneed to be calibrated. For calibration, a method as explained furtherbelow can be used.

However, there is also an other way that may be faster and moreaccurate, which is described in the following. We emphasize that thesubsequently proposed calibration procedure neither need any additionalhardware in BS, nor any standard changes.

Basically, every antenna array is provided with an equipment formodifying the baseband signal so that the signal leaving the antennaarray to the physical layer, i.e. the air interface in radiotransmission systems, is known. When there are two transmit antennas andalso two transmit chains, this means that two basebands, two D/Aconverter, two power amplifiers etc. are provided, thus establishing twoRF chains, one chain per antenna.

However, even though each chain is similar in design, the usedcomponents are actually slightly different because of their non-idealcharacteristics. Hence, also the signal leaving the antenna system isnot the same per antenna, even if controlled by the identical controlsignals. As a consequence, signal phase between the signals of thechains is not preserved in the RF chains. Accordingly, a phasecompensation by means of applying a transmit weight at the baseband sidehas to be effected for compensating the phase error. Determination ofthe weight to be applied represents the calibration problem.

To this end, just before the antenna, a receiver means (still at thetransmitter) is provided, which measures the transmitter's own signal.Based on this, the non-idealities are determined and the weight forcompensation is deduced.

-   -   For the proposed calibration, such receivers are selected for        which the channel preserves well the polarization. This will        make the calibration more reliable. If there are none, then the        enhanced class 2 is not used (it is not feasible) and        calibration is not necessarily needed. (Preservation of        polarization can be recognized from the received power ratio        p1/p2.)    -   By averaging received signal powers and relative phases between        polarization branches, suitable transmit weights for downlink        transmission are found (for each link separately). But these        weights are not used immediately.    -   Then, comparison is effected of (a) the transmit weights that        are formed using received feedback commands and (b) transmit        weights that are formed using polarization measurements. The        possible difference in the transmit weights is the error that        needs to be compensated by calibration. Since several receivers        (at least one) is connected into the calibration process, the        resulting calibration constants (same for all mobiles) are        obtained using the average differences in transmit weights. This        calibration procedure is fast and no additional hardware is        needed. If closed loop diversity mode class 1 is employed, then        the relative phase between transmit chains can be calibrated as        explained. Relative transmit power can be calibrated when class        2 is employed, at least for some links.

For example, if closed-loop mode class 1 is employed, we can do thecalibration as follows: assume that K receivers are Connected to thecalibration. Receivers are selected such that correlation betweenantennas is high. This is, for example, the case if there is strongline-of-sight (LOS) component in the received signal. For the relativephases there holdsΔφ_(UL,ANT) ^(k)=Δφ_(UL,DSP) ^(k) +C _(UL)Δφ_(DL,ANT) ^(k)=Δφ_(DL,DSP) ^(k) +C _(DL)  (1)

Where left side of the equation is the relative phase between signalswhen signals leave the antennas, first term in the right side of theequation is the relative phase of signals seen in digital signalprocessing (DSP) unit before transmission and the last term in the rightis the calibration constant. First equation corresponds to the receiverchains while second equation corresponds to the transmitter chains.Index k refers to the receiver. If antennas are correlated we shouldhaveΔφ_(UL,ANT) ^(k)Δφ_(DL,ANT) ^(k)  (2)

Then transmitted signals are summed coherently in the direction of thereceiver k. If closed-loop mode class 1 is used this equation isapproximately true. Then we obtainΔφ_(UL,DSP) ^(k)−Δφ_(DL,DSP) ^(k)=Δφ_(UL,ANT) ^(k)−Δφ_(DL,ANT) ^(k) +C_(DL) −C _(UL)  (3)

Now the difference between calibration constants is estimated usingequation

$\begin{matrix}{{C_{UL} - C_{DL}} \approx {\frac{1}{K}{\sum\limits_{k = 1}^{K}\left( {{\Delta\phi}_{{DL},{DSP}}^{k} - {\Delta\phi}_{{UL},{DSP}}^{k}} \right)}}} & (4)\end{matrix}$

After estimating the difference between calibration constants we canselect the relative phase for transmission using the equationΔφ_(DL,DSP) ^(k)=Δφ_(UL,DSP) ^(k) +C _(UL) −C _(DL)  (5)

Hence, only UL measurements are needed any more. Calibration needs to beupdated once and a while. Update time is system related.

FIG. 4 shows a transmitter implementing the present invention, i.e. themethod for diversity mode selection, the method for controlling the modeto an enhanced mode, and the calibration method. However, please notethat details of implementation are omitted and that FIG. 4 shows merelya basic functional block diagram, while even omitting details concerningthe calibration process in order to keep the description simple.

As shown in FIG. 4, the transmitter comprises a look-up table LUT, whichitself consists of a plurality of look-up table performance charts LUT1,LUT2, LUT3. For example, LUT3 and LUT2 may have a contents asrepresented in FIGS. 3 and 2, respectively, while LUT1 may have acontents basically as represented in FIG. 1. (Note that FIG. 1illustrates a case for P1/P2, while LUT1 in FIG. 4 is intended to beused for a range of P1/P2=0 . . . 3 dB). Further look-up tableperformance charts may be used and/or the power ratio ranges may varyfrom those indicated in FIG. 4. Furthermore, the transmitter comprisestwo diversity antennas Ant1 and Ant2, each preceded by an RF chain RF1,and RF2, respectively.

The RF chains are controlled by applying at least a weight signal,output from a weight selection means. The weight selection means is atleast controlled by a control signal output from a control means CTRL.The output of the control means CTRL is dependent on feedbackinformation received from a receiver (not shown). (Basically, thisweight control arrangement as described so far corresponds to priorart).

The control means receives the feedback information and determines acontrol signal for weight selection. In addition, the control means alsodetermines the ratio of received powers P1/P2 and the space correlationSC and time correlation TC values. This determination can be based onthe received feedback information and determined by calculation, or maybe based on uplink measurements.

The determined ratio P1/P2 is applied for controlling the selection ofone of the look-up table performance charts LUT1 to LUT3 dependent onP1/P2. (This is indicated by the controlled switch controlled by P1/P2.)Once a look-up up table performance chart is selected, the parameters TCand SC are applied to select the appropriate diversity mode therefrom,indicated in the performance chart for this parameter pair. The thusselected diversity mode is then activated by applying a diversity modecontrol signal DMC to the RF chains. (Note that the selection order canalso be exchanged, i.e. first TC/SC selects in each of LUT1 to LUT3 amode to be selected, and one of the thus pre-selected modes is thenselected based on P1/P2.) Additionally, the power ratio P1/P2 may alsobe applied for weight selection, as shown in FIG. 4.

Accordingly, as has been described herein before, the present inventionconcerns a method for selecting a diversity mode A, B, C to be appliedby a transmitter having two cross-polarized antenna arrays Ant1, Ant 2,each representing a diversity branch, for transmission diversity, themethod comprising the steps of: providing S10 a plurality of diversitymode performance chart look-up up tables LUT, LUT1, LUT2, LUT3, eachperformance chart look-up table mapping a respective individualdiversity mode A, B, C out of a plurality of individual diversity modesto a respective pair of time correlation value TC and space correlationvalue SC for said two cross-polarized polarized antenna array beams,wherein a respective individual diversity mode is presented by a mappingarea, wherein the plurality of performance chart look-up tables isparameterized by an indication of a ratio P1/P2 of received powers fromsaid diversity branches, and the mapping is different for differentperformance charts, first determining S11 the ratio of received powersfrom said diversity branches, second determining S12 the actual timecorrelation and space correlation for said pair of two cross-polarizedantenna arrays, first selecting S13 one of said performance chartlook-up tables dependent on determined ratio P1/P2 of received powersfrom separate beams, and second selecting S14 one of said individualdiversity modes A, B, C according to the mapping to the determinedactual time correlation TC and space correlation SC values from saidfirst selected performance chart look-up table.

While the invention has been described with reference to a preferredembodiment, the description is illustrative of the invention and is notto be construed as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

1. A method for selecting a diversity mode (A, B, C) to be applied by atransmitter having two cross-polarized antenna arrays (Ant1, Ant 2),each representing a diversity branch, for transmission diversity, themethod comprising the steps of: providing (S10) a plurality of diversitymode performance chart look-up tables (LUT, LUT1, LUT2, LUT3), eachperformance chart look-up table mapping a respective individualdiversity mode (A, B, C) out of a plurality of individual diversitymodes to a respective pair of time correlation value (TC) and spacecorrelation value (SC) for said two cross-polarized antenna array beams,wherein a respective individual diversity mode is presented by a mappingarea, wherein the plurality of performance chart look-up tables isparameterized by an indication of a ratio (P1/P2) of received powersfrom said diversity branches, and the mapping is different for differentperformance charts, first determining (S11) the ratio of received powersfrom said diversity branches, second determining (S12) the actual timecorrelation and space correlation for said pair of two cross-polarizedantenna arrays, first selecting (S13) one of said performance chartlook-up tables dependent on determined ratio (P1/P2) of received powersfrom separate beams, and second selecting (S14) one of said individualdiversity modes (A, B, C) according to the mapping to the determinedactual time correlation (TC) and space correlation (SC) values from saidfirst selected performance chart look-up table.
 2. A method according toclaim 1, wherein said selected diversity mode is selected for eachindividual link established by the transmitter.
 3. A method according toclaim 1, wherein said diversity modes are classified as open-loopdiversity modes (A) and closed-loop diversity modes (B, C), and saiddetermined ratio of received powers is applied as a further controlparameter for controlling said closed-loop diversity modes whenactivated upon selection.
 4. A method according to claim 1, wherein saidfirst determining and said second determining are performed at saidtransmitter.
 5. A method according to claim 1, wherein providing saidperformance chart look-up tables is effected beforehand based onsimulation results and/or measurement cycles.
 6. A method according toclaim 1, wherein both arrays consists of one antenna element and antennacalibration in the transmitter is performed by using both the feedbackfrom said receiver according to one of the closed-loop modes, and thereceived signals from cross-polarized antenna arrays in the transmitter.7. A method according to claim 1, wherein said mapping of diversitymodes differs for different performance chart look-up tables dependenton the determined ratio of received powers from said diversity branches.8. A method according to claim 7, wherein said diversity modes areclassified as open-loop diversity modes and closed-loop diversity modes,and a mapping area of at least one closed-loop diversity mode (C)increases (FIG. 2, FIG. 3) dependent on the indication of a ratio ofreceived powers from said diversity branches.